Ghostly Hum caused by Triamp 3 Panel Lighting

This one sounds a little surprising and I wonder if anyone has seen it before...

While I was setting up my newly purchased Triamp3 with a G-System (4-cable method) I could hear a ghostly hum at about 392Hz (the same frequency as a G played on the top E string of the guitar). After checking my cables and power I finally found that the ghostly hum went away when I turned the Triamp 3 panel brightness down to zero! There’s still the usual (acceptable) levels of electronic noise but the ghostly hum has gone.

The hum can’t be heard when the G-Systen is not connected so it seems to be something to do with the Send-Return loop. I should also point out that the hum is not heard on any of my other amps, so it is a combination of the Triamp 3 panel lighting and connecting an FX system. The hum is also there when the FX loop is switched off, but disappears if I remove any cable connecting the G-System to the Triamp 3. Disconnecting the guitar cable from the G-System makes no difference.

The noise gate did mask the issue but the hum could still be heard when I played quietly, so wasn’t really a solution. The noise gate on the G-System worked better as there are more control options on the G-System noise gate. The Triamp 3 noise gate even at its softest still has a relatively hard cut-off. On the G-System I can select the dB level for cut-off.

Has anyone observed this before? It isn’t a showstopper for me with the head - and even seems somewhat amusing to me that a ghostly hum can be heard with the ghostly blue light. But if you know an easy fix that will let me keep the panel lights on when I use the head at home then that would be great. I’m hoping it won’t be an issue live as it would be nice to have them on for shows - and to see the (many) controls.

Not really a clue as to what is going on yet, the schematics don't seem to show the lighting so I can't see where it is being fed from and what else would affect it. However, the signal path selections in the Triamp seem to be made through opto-isolators in the signal path. If something starts to put noise onto their control voltages then you will get noise in the signal path. I'm intrigued and still looking.